Installing ESXi 5.0 requires a boot device that is a minimum of 1GB in size.

When booting from a local disk or SAN/iSCSI LUN, a 5.2GB disk is required to allow for the creation of the VMFS volume and a 4GB scratch partition on the boot device.

If a smaller disk or LUN is used, the installer will attempt to allocate a scratch region on a separate local disk.

If a local disk cannot be found the scratch partition, /scratch, will be located on the ESXi host ramdisk, linked to /tmp/scratch.

You can reconfigure /scratch to use a separate disk or LUN. For best performance and memory optimization, VMware recommends that you do not leave /scratch on the ESXi host ramdisk.

To reconfigure /scratch, see Set the Scratch Partition from the vSphere Client.

Due to the I/O sensitivity of USB and SD devices the installer does not create a scratch partition on these devices. As such, there is no tangible benefit to using large USB/SD devices as ESXi uses only the first 1GB.

When installing on USB or SD devices, the installer attempts to allocate a scratch region on an available local disk or datastore.

If no local disk or datastore is found, /scratch is placed on the ramdisk. You should reconfigure /scratch to use a persistent datastore following the installation.

It would follow the same procedure as any install or upgrades, to the infrastructure it acts all the same.

Can an ESXi Host access USB devices ie. Can an External USB Hard Disk be connected directly to the ESXi Host for copying of data?

Yes this can be done, see the KB below – ‘Accessing USB storage and other USB devices from the service console’

However the technology that supports USB device pass-through from an ESX/ESXi host to a virtual machine does not support simultaneous USB device connections from USB pass-through and from the service console.

This means the host is in either Pass Through (to the VM) or service console mode.